1. Field of the Invention
The present invention is directed to a fire alarm system, and more particularly to a fire alarm system in which a receiver receives the information of the smoke density detected at each smoke detector to produce an alarm signal when the smoke density of significant level lasts over a predetermined reference time period.
2. Description of the Prior Art
There has been proposed a variety of fire alarm systems utilizing a plurality of smoke detectors to be installed in different locations for monitoring the smoke density indicative of fire presence or absence in each location. Such systems have been generally designed to produce an alarm when any one of the smoke detectors sees a serious smoke density. However, in these systems, only the smoke density is utilized as a criterion for determining fire presence or absence as well as a fixed reference smoke density is utilized as the criterion, which frequently results in a false fire detection as in the case when the smoke detector or detectors are installed in those rooms where there is frequent smoking or use of cooking appliances. To make matters worse, in the case that the smoke detector is adapted to operate fire prevention equipment such as a fire door, fire extinguisher and smoke ejector upon the detection of smoke density exceeding the reference density, the false detection of fire will require restoring the equipment to its initial condition. This actuation of the fire equipment by mistake or due to false fire detection is most disadvantageous when occurring at night. But unfortunately, in the prior art there is no effective scheme for preventing the malfunction at night other than disconnecting the operation of the system at night, which would therefore be no more effective for true fire occurrence at night and be very dangerous.